Two-stroke-cycle engine with variable valve timing

ABSTRACT

A two-stroke-cycle spark ignited internal combustion engine operates with an exhaust valve that is controlled independently of crankshaft position and optimally for high power and low pollutant output in combination with a scavenging pump and fuel injector. The volume of combustible mixture is established at the point in the cycle when the exhaust valve is selectably closed with the piston traveling upwardly and decreasing the volume of the cylinder. Throttling losses are eliminated since the piston forces the scavenging air out of the cylinder through the wide-open exhaust valve rather than the air being drawn into the cylinder against the reduced pressure caused by a conventional partially closed throttle plate. The volume of combustible air in the cylinder is determined by the point in the cycle at which the exhaust valve is closed and a correspondingly appropriate charge of fuel is thereafter injected into the cylinder.

SUMMARY OF THE INVENTION

The present invention relates generally to internal engines and moreparticularly to such internal combustion engines of the spark-ignitedtype and ones which operate in a two-stroke-cycle mode. Specifically,the present invention relates to variable timing of the valves, andparticularly the exhaust valve, in such a two-stroke-cycle spark-ignitedinternal combustion engine.

Internal combustion engine valves in four-stroke cycle engines arealmost universally of a poppet type which are spring loaded toward avalve-closed position and opened against that spring bias by a cam on arotating cam shaft with the cam shaft being synchronized with the enginecrankshaft to achieve opening and closing at fixed preferred times inthe engine cycle. This fixed timing is a compromise between the timingbest suited for high engine speed and the timing best suited to lowerspeeds or engine idling speed. The valves in two-stroke-cycle enginesare generally simple apertures or ports in the cylinder sidewall whichare uncovered or opened by piston movement, however, exhaust valving ofthe cam actuated as well as other varieties have been suggested.

A two stroke-cycle compression ignited (Diesel) engine utilizing aconventional cam actuated overhead valve as the exhaust valve with thetraditional cylinder sidewall intake ports receiving pressurizedscavenging air from a positive displacement (Roots) blower is know. Theexhaust valving of this known Diesel engine suffers from the abovedefects, but when operated over a narrow range of speeds, it operateswith relatively high efficiency since there are little or no throttlinglosses in its operation. It would be highly desirable to be able tooperate a spark-ignited two-stroke-cycle engine over a wide range ofspeeds with little or no throttling losses, but up until now this hasnot been possible because such spark-ignited engines require a fuel toair ratio mix versus retained exhaust gas within a fairly narrow rangeof values for successful ignition. Control of such an engine, then,requires some measure of control over both the quantity of fuel enteringthe cylinder and a control of the quantity of air entering the cylinderas well as the quantity of retained exhaust gas. The control of thequantity of air entering the cylinder has, up until now, been controlledby a restriction or throttling of the air path into the cylinder againstwhich piston motion had to work to suck the desired quantity of air intothe cylinder. Such throttling has been so commonplace that thetraditional name attached to the engine speed control in aircraft,boats, steam engines and many other craft is "throttle."

A two stroke-cycle spark-ignited engine utilizing a conventionalignition system and having a fuel injector which introduces a controlledquantity of fuel directly into the closed end of the cylinder cavity hasalso been proposed. This engine utilizes the traditional cylindersidewall intake ports receiving pressurized scavenging air from apositive displacement blower and cylinder sidewall exhaust ports which,in addition to being opened and closed by piston travel, are valved byrotary exhaust valves. The exhaust valving of this known Otto cycleengine appear to be either chain or cam driven, but, in either case,appears to be fixed in its timing and to suffer from the above defects.

The prior art has recognized numerous advantages which might be achievedby replacing such cam actuated or similar valve arrangements with othertypes of valve opening mechanism which could be controlled in theiropening and closing as a function of engine speed as well as enginecrankshaft angular position or other engine parameters.

For example, in U.S. Pat. application Ser. No. 226,418 entitled VEHICLEMANAGEMENT COMPUTER filed in the name of William E. Richeson on July 29,1988 there is disclosed a computer control system which receives aplurality of engine operation sensor inputs and in turn controls aplurality of engine operating parameters including ignition timing andthe time in each cycle of the opening and closing of the intake andexhaust valves among others. This application teaches numerous operatingmodes or cycles in addition to the conventional four-stroke cycle. Inparticular, this application discloses the principles suitable forimplementing a control computer for the two-stroke-cycle engine of thepresent invention.

In copending application Ser. No. 153,257, now U.S. Pat. No. 4,878,464,entitled PNEUMATIC ELECTRONIC VALVE ACTUATOR, filed Feb. 8, 1988 in thenames of William E. Richeson and Frederick L. Erickson and assigned tothe assignee of the present application there is disclosed a valveactuating device which is a jointly pneumatically andelectromagnetically powered valve with high pressure air supply andcontrol valving to use the air for both damping and as one motive force.A magnetic motive force is supplied from the magnetic latch opposite theone being released and this magnetic force attracts an armature of thedevice so long as the magnetic field of the first latch is in itsreduced state. As the armature closes on the opposite latch, themagnetic attraction increases and overpowers that of the first latchregardless of whether it remains in the reduced state or not. Thiscopending application also discloses different operating modes includingdelayed intake valve closure and a six stroke cycle mode of operation.

The forgoing as well as a number of other related applications allassigned to the assignee of the present invention and filed in the nameof William E. Richeson or William E. Richeson and Frederick L. Ericksonare summarized in the introductory portions of copending Ser. No.07/294,728 filed in the names of Richeson and Erickson on Jan. 6, 1989and entitled ENHANCED EFFICIENCY VALVE ACTUATOR. Any of the valveactuators disclosed in these applications may be advantageously utilizedin implementing the two-stroke-cycle engine of the present invention.

The entire disclosures of all of the above identified copendingapplications are specifically incorporated herein by reference.

Among the several objects of the present invention may be noted theprovision of a spark-ignited engine operable in a two-stroke-cycle modewithout the throttling losses heretofor characteristic of suchspark-ignited engines; the provision of a two-stroke-cycle spark-ignitedengine having the advantages of a two-stroke-cycle diesel engine, butoperable efficiently over a wide speed range, the provision of an enginein accordance with the previous object which may be incorporated intolarge vehicles to facilitate a reduction in the complexity of thetransmissions thereof; the provision of an unthrottled spark-ignitedengine; the provision of an engine having an exhaust valve which closesat a time in the engine cycle which is determined by the power demandedfrom the engine; the provision of a cleaner burning two-stroke-cycleengine as compared to most of the current two or four stroke enginescurrently available; and the provision of a unique control technique fora two-stroke-cycle internal combustion engine. These as well as otherobjects and advantageous features of the present invention will be inpart apparent and in part pointed out hereinafter.

It has long been a goal in engine design to achieve a lean burn. Leanburns burn slowly and to get the most useful burn at a given RPM theignition typically takes place at an early time during the compressionstroke. Successful ignition depends on the air/fuel mix and the densityand temperature of the ignition plasma. The lean burn achievable withthe present two-stroke-cycle engine can be used to reduce emissions. Dueto the stratified charge in the engine cylinder, a rapid lean burnfollowed by a rapid cooling of the combusted gases is possible. A morethorough burning can take place which will reduce unburned hydrocarbonand carbon monoxide emissions. The subsequent rapid cooling of thecylinder gas reduces the maximum burn temperature which in turn reducesNOX emissions.

Stratification of charge after a thorough purging of the combustionchamber of the present inventive engine yields further unsuspectedadvantages. The exhaust gases that are normally emitted near the end ofthe exhaust stroke are rich in unburned hydrocarbons due to scavengingeffects of the unburned boundary layers close to the cooler combustionchamber walls and the boiling of unburned hydrocarbons out of cavitiessuch as around the head gasket and around the piston and its compressionrings that were deposited there due to pressurization of the charge dueto the compression stroke and burning charge pressurization. With thepresent technique, it is fuel-free air which is compressed around therings and piston head. Moreover, the air flow from the lowermost portionof the cylinder adjacent the head of the piston upwardly through thecylinder and out the exhaust valve in the dome of the cylinder head ishighly effective in purging the cylinder of all the combustion products.These effects combine to reduce the likelihood of detonation at highcompression ratios (knock) and therefore allow the use of lower octanefuels.

Therefore, further objects of the present invention are to provide anengine of reduced unburned hydrocarbon, NOX and carbon monoxideconcentration in the exhaust; an engine which allows the utilization ofreduced octane fuels; and such an engine which will go toward greatlyreducing or eliminating the need for the catalytic converter.

In general, a method of operating a two-stroke-cycle internal combustionengine at a controlled fuel to air mixture ratio includes varying thetime in the cycle at which an exhaust valve is closed under varying loadconditions and correspondingly varying the quantity of fuel introducedinto the engine so as to maintain the controlled ratio of fuel to air inthe combustion chamber at the time of ignition. Typically the exhaustvalve is open longer during each cycle under reduced load conditions andis closed at a time in the cycle to entrap a quantity of air appropriateto operation at a desired engine speed. An amount of fuel appropriate tothat desired engine speed is subsequently introduced into the entrappedquantity of air.

Also in general and in one form of the invention, a method of operatinga spark-ignited internal combustion engine includes injecting fuel intoa cylinder of the engine at a time in the engine cycle which varies withthe demand placed on the engine, however, this injection of fuel isalways a fixed rotational increment after closure of the cylinderexhaust valve or that same fixed rotational increment after closure ofthe cylinder intake port whichever is later in the cycle. The quantityof fuel to be ingested into the engine is selected in accordance withthe time at which the exhaust valve is closed so as to maintain adesired fuel to air ratio. Supercharging of the combustion air as itenters an engine cylinder may be achieved by maintaining a relativelyconstant pressure air source at a cylinder inlet and opening thatcylinder inlet a fixed percentage of each engine cycle; and closing anexhaust valve of the cylinder at any selected time while the intakevalve is open.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view through one engine cylinder with thepiston in its lowermost or bottom dead center position and illustratingthe invention in one form;

FIG. 1a is a cross-sectional view of the upper portion of the cylinderof FIG. 1 in a plane orthogonal to the plane of FIG. 1;

FIG. 2 is a view similar to FIG. 1, but showing the piston after it hasmoved upward just sufficiently to close the air intake ports in thecylinder sidewalls;

FIG. 3 is a view similar to FIG. 1, but showing the piston 90 degreesbefore top center;

FIG. 4 is a view similar to FIG. 1, but showing the piston 75 degreesbefore top center;

FIG. 5 is a view similar to FIG. 1, but showing the piston 45 degreesbefore top center;

FIG. 6 is a view similar to FIG. 1, but showing the piston 30 degreesbefore top center;

FIG. 7 is a view similar to FIG. 1, but showing the piston at topcenter;

FIG. 8 is a view similar to FIG. 1, but showing the piston beyond topcenter and about to uncover the cylinder sidewall inlet ports;

FIG. 9 is a view similar to FIG. 1, but showing the piston beyond topcenter just after it has uncovered the cylinder sidewall inlet ports;

FIG. 10 is an illustrative engine timing diagram for the engine of FIGS.1-9 for a low torque, low speed condition;

FIG. 11 is an illustrative engine timing diagram similar to FIG. 9, butfor a low torque, medium speed condition;

FIG. 12 is an illustrative engine timing diagram similar to FIG. 9, butfor a low torque, high speed condition;

FIG. 13 is an illustrative engine timing diagram similar to FIG. 9, butfor a high torque, low speed condition;

FIG. 14 is an illustrative engine timing diagram similar to FIG. 9, butfor a high torque, medium speed condition;

FIG. 15 is an illustrative engine timing diagram similar to FIG. 9, butfor a high torque, high speed condition; and

FIG. 16 is a schematic illustration of a computer control for anillustrative four cylinder engine.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawing.

The exemplifications set out herein illustrate a preferred embodiment ofthe invention in one form thereof and such exemplifications are not tobe construed as limiting the scope of the disclosure or the scope of theinvention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, a two-stroke-cycle internal combustion enginehas a crankshaft 11 with conventional counterbalancing 13 and connectingrod 15 coupled to a piston 17. While only one cylinder 19 is illustratedin FIG. 1, it will be understood that each cylinder of a typicallymulti-cylinder engine has common features such as an exhaust poppetvalve 21 for each cylinder of the engine along with a valve actuator 23which is operable on a first command as from a computer control 25 (FIG.16) to open the cylinder poppet valve as well as being operable on asecond command to close a cylinder poppet valve. The computer controlselectively determines the time of occurrence of the second command inaccordance with a desired engine output characteristic. Thetwo-stroke-cycle internal combustion engine will typically be operatedat a controlled fuel to air mixture ratio by varying the time in thecycle at which the exhaust valve 21 is closed under varying loadconditions as determined by vehicle 27 and operator 29 inputs, andcorrespondingly varying the quantity of fuel introduced into the engineby fuel injectors such as 30 and 32 located in the engine head so as tomaintain the controlled ratio of fuel to air in the piston or combustionchamber 19 at the time of ignition. Typically, the exhaust valve will beopen longer and, therefore, a lesser quantity of air will be retained inthe cylinder during each cycle under reduced load conditions. Thus, thetime or rotational position in the cycle at which the exhaust valveshould be closed to trap an optimum quantity of air for a given enginepower level is first determined and then the quantity of fuel to beingested is selected accordingly to maintain a desired combustible fuelto air ratio.

Each combustion chamber or cylinder 19 has a series of openings 31 whichare selectively opened and closed by piston motion. These openings 31communicate with a source of pressurized combustion air 33 such as apositive displacement pump 35 which is coupled to the holes 31. Theholes function as an intake port for supplying air to the cylinder whilethe piston 17 is in a position (down as viewed) to open this intakeport. As shown, sump 41 is an actual oil sump for lubricating the enginebearings, but it may, in relatively small engines, provide the functionof pumping air into the cylinder on the down stroke of the piston as isknown in two-stroke-cycle engines if desired.

Precise control of the time of opening and closing of the illustrativeexhaust valve 21 by valve actuator 23 allows attainment of the loftygoals of the present invention. FIG. 1 illustrates the engine mechanismat bottom center with the crankshaft 11 turning clockwise as viewed. Atthis point in the cycle, the intake ports 31 are wide open and thecylinder or combustion chamber 19 is being scavenged of the exhaustgases by a Roots type positive displacement blower 35. The scavenging(fresh) air enters the lower ports 31 which extend about the peripheryof the cylinder sidewall and sweeps upwardly through the cylinder andcarries these gases out through the exhaust port of the open exhaustvalve 21. If high power at low speed is desired, the exhaust valve 21may be closed with the piston 17 close to bottom dead center. This willallow the maximum entrapment of fresh air and some supercharging whilethe inlet ports 31 are still open. If high power at high speed isdesired, the exhaust valve 21 may be closed when the piston has justcovered the intake (inlet) ports 31, i.e., the piston position of FIG.2. At this point, the scavenging is completed at high speed and theentrapped air will occupy all of the cylinder volume above the inletports. Once the exhaust valve 21 has closed, the fuel injector 30 willfire a proportionate amount of atomized fuel into the cylinder. The timeof such fuel injection is shown by an * in FIGS. 10-15 and occursgenerally at a point in the engine cycle which varies with the demandplaced on the engine yet always a fixed rotational increment afterclosure of the cylinder exhaust valve or that same fixed rotationalincrement after closure of the cylinder intake port whichever is laterin the cycle. Thus, in FIGS. 10, 11 and 12, the low torque conditions,fuel injection occurs a fixed angle after the exhaust valve closes whilefor the high torque conditions of FIGS. 13-15 the fixed angle is afterclosure of the intake port. The time of ignition on the other hand is anearly constant angle prior to top dead center as shown by the O withsome spark advance occurring with increasing engine speed.

The time in the cycle at which an exhaust valve is closed under varyingload and engine speed conditions and therefore also the quantity of fuelintroduced into the engine combustion chamber are both varied so as tomaintain a desired ratio of fuel to air in the combustion chamber at thetime of ignition. Generally speaking, the exhaust valve is open longerduring each cycle under reduced load conditions. The exhaust valve isclosed at a point in the cycle to entrap a quantity of air appropriateto operation at a desired engine speed and an amount of fuel appropriateto that desired engine speed is subsequently introduced into theentrapped quantity of air. The amount of atomized fuel injected into thecylinder is selected, as determined at least in part by the time ofclosing of the exhaust valve, to always provide a controlled air/fuelratio to the combustion chamber. If lower power is desired, the exhaustvalve is simply left open longer to allow the piston to push more airout of the exhaust port so that when the exhaust valve does close asmaller volume of air is entrapped. A correspondingly smaller volume offuel is then injected.

FIG. 3 illustrates a point at which the exhaust valve is just beginningto close with FIG. 4 showing that closure completed. The air entrappedabove the piston in FIG. 4 results in a somewhat medium power condition.In comparing FIGS. 4 and 5, it will be noted that the valve 21 has beenretarded in FIG. 5 so as to close later in the cycle when the piston 17has traveled upwardly expelling additional air. FIG. 5 illustrates thecondition where very low power or close to idle conditions are required.FIGS. 5 and 6 span this low power condition with FIG. 6 showingcompletion of valve closure. A very small amount of air will beentrapped above the piston of FIG. 6. It should be noted that during theentire air induction process and the precise metering of entrapped airby ejecting air through the exhaust port until that valve closes, therehas been a total absence of throttling, hence, a total absence ofthrottling losses. Because the exhaust valve may be closed at selectabletimes, a moderate compression spark ignited engine having the nearlyzero throttling losses heretofor attainable only with certain types ofcompression ignition engines is now possible.

FIG. 7 shows the piston at top dead center position just after ignition.FIG. 8 shows the piston returning downwardly to a typical location wherethe exhaust valve is reopened to allow exhaust blowdown to substantiallyatmospheric pressure in the combustion chamber (or at least to apressure below that of the fresh air at the intake ports) prior to theopening of those intake ports as shown in FIG. 9.

The engine management computer 25 may select the timing angle at whichthe exhaust valve opens and the angle at which it closes giving theengine of the present invention a wide range of operational flexibility.The timing diagrams of FIGS. 10-12 illustrate slightly different pointsat which the exhaust valve opens (EXOP) and closes (EXCL) with thatvalve closing progressively earlier in the cycle as the engine speedincreases from 500 RPM for FIG. 10, 1500 RPM for FIG. 11 to 4500 RPM forFIG. 12. The EXCL points determine three specific volumes which willyield three different power levels with the power level beingproportional to the amount of air entrapped by valve closure. Lateclosing, of course, corresponds to relatively small quantities of airand light torque conditions. The exhaust valve is opened (EXOP) in allsix timing diagrams progressively earlier as engine speed increases.Note in these same figures the opening of the exhaust valve varies tomaximize the expansion ratio and efficiency as well as to optimizeexhaust evacuation. Opening timing should be such that the cylinderpressure at the time the intake air ports opens should be equal to orlower than the intake air pressure.

FIGS. 10 and 13 illustrate that at low speed the angle between theexhaust valve opening and inlet air opening (INOP) can be set very closeto one another because the combustion chamber has more time due to theslow speed condition to blow down to the inlet port pressure at 43. Athigh operational speeds as shown in FIG. 12 and 15, the angular lead ofthe exhaust valve opening prior to the inlet being uncovered must begreater to allow sufficient time for the exhaust pressure to blow downto the inlet blower pressure.

FIGS. 13-15 show high torque conditions as the engine speed increasesfrom 500 RPM for FIG. 13, 1500 RPM for FIG. 14 to 4500 RPM for FIG. 15.FIG. 13 illustrates that the exhaust valve can be closed as early asbottom center since there is adequate time for the scavenging functionat low engine speed. Between the time the exhaust valve closes and thetime the inlet valve closes (INCL) pump 35 is forcing air under pressureinto combustion chamber 19 causing a certain amount of supercharging.FIGS. 14 and 15 are illustrative of high torque at mid and high speedconditions respectively. Note that as the engine speed increases so alsodoes the angle between the opening of the exhaust valve and the openingof the inlet port. For fixed inlet port timing, the exhaust valve isopened progressively earlier with increasing speed in order to allowsufficient time for exhaust pressure to blow down to the inlet portpressure. Similarly, as the speed increases, the exhaust valve remainsopen longer to allow the blower to force all the combustion gases fromthe combustion chamber.

As is clear from an inspection of FIG. 16, the illustrative cylinder ofFIGS. 1-9 will frequently be in a two-stroke-cycle engine having amultiplicity of pistons reciprocable in respective cylinders wherein thecycle for each cylinder has intake, exhaust and compression segments asdepicted in FIGS. 10-15. Moreover, such a multicylinder engine will havean arrangement which includes at least one exhaust valve for each saidcylinder that is opened and closed by a respective valve actuator toobtain means for adjusting each of said exhaust segments to provide acompression ratio in each of said cylinders which has a predeterminedrelationship to engine torque and speed. As shown in FIGS. 10-15, thecompression ratio for low torque is significantly lower than that forhigh torque and the compression ratio for low speed is less than forhigh speed. Moreover, the vehicle computer 25 senses the engine torqueand speed demands and provides control signals to each said valveactuator to cause said actuator to open and close its respective valveto obtain said compression ratios. The computer functions as a controlmeans in response to engine load and engine speed to vary the exhaustsegment of the cycle accordingly for opening the exhaust valve prior tothe intake segment of the cycle and closing the exhaust valve at thecompletion of the exhaust segment whereby a sufficient charge of air isadmitted into the cylinder to have substantially purged the cylinder ofthe burned gases from the previous ignition segment of the cycle and tointroduce substantially clean air into said cylinder.

Each cylinder of such a two-stroke-cycle engine will typically have apiston 17 with a plurality of piston rings such as 18 and 20reciprocable in the closed ended cylinder 19 and the typical cycle willhave intake, exhaust, compression and ignition segments. Each cylinderwill have a fuel injector 30 located near the closed end to admit fuelinto the cylinder after the exhaust segment of the cycle as well as aspark igniter 37 located near the closed end. The intake port or ports31 are located in the cylinder sidewall and the pump 35 supplies acharge of pressurized air through the intake port 31 into the cylinder19 during the intake segment of the cycle. The spark igniter 37 isenergized after the fuel injector 30 has admitted fuel into the cylinderto provide in concert with the exhaust segment stratified fuel/airlayers in said cylinder with the richest fuel/air layer near the upperclosed end of the cylinder and progressively less rich fuel/air layersaway from the closed end toward the piston 17 with a layer ofpredominantly fresh air covering the piston and piston rings 18 and 20whereby the piston and piston rings are cleansed of previously burnedgases and the combustion gases are quickly cooled by the progressivelyless rich and fresh air layers and the fresh air layer mixes quicklywith the already ignited richer fuel/air layers due to the gaseousturbulence caused by the extreme temperature gradient after ignition tominimize hydrocarbon and NOX pollutants in the cylinder exhaust gases.

The overall control system for the present two-stroke-cycle engineemploys substantially the same principles as disclosed in theabovementioned U.S. Pat. application Ser. No. 226,418 entitled VEHICLEMANAGEMENT COMPUTER, but differs therefrom in utilizing but a singlevalve actuator for each engine cylinder. Such a system is shownschematically in FIG. 16. The control computer 25 receives operatordemand input signals on line 29 and input information on the currentoperation of the engine such as engine speed (RPM) on line 27. A sourceof high pressure air 33 is provided for powering the valve actuatorssuch as 23 which may, for example, be of any one of the types summarizedin the abovementioned ENHANCED EFFICIENCY VALVE ACTUATOR. The computeralso issues commands for actuating the fuel injectors such as 30 and 32and the ignition system 39. As shown in FIG. 16, there is one valveactuator, one fuel injector and one spark plug such as 37 for each oneof four different engine cylinders.

From the foregoing, it is now apparent that a novel two-stroke-cycleengine as well as a novel arrangement for varying the time of openingand closing an exhaust valve in such a two-stroke-cycle engine have beendisclosed meeting the objects and advantageous features set outhereinbefore as well as others, and that numerous modifications as tothe precise shapes, configurations and details may be made by thosehaving ordinary skill in the art without departing from the spirit ofthe invention or the scope thereof as set out by the claims whichfollow.

What is claimed is:
 1. The method of operating a two-stroke-cycleinternal combustion engine, having a two-stroke engine cycle, at acontrollable fuel to air mixture ratio comprising varying the time inthe cycle at which an exhaust valve is closed under varying load andengine speed conditions and correspondingly varying the quantity of fuelintroduced into the engine combustion chamber so as to maintain adesired ratio of fuel to air in the combustion chamber at the time ofignition, the exhaust valve being closed at a time in the cycle toentrap a quantity of air appropriate to operation at a desired enginespeed and an amount of fuel appropriate to that desired engine speedbeing subsequently introduced into the entrapped quantity of air.
 2. Themethod of operating a two-stroke-cycle internal combustion engine ofclaim 1 wherein the exhaust valve is open longer during each cycle underreduced load conditions.
 3. The method of operating a two-stroke-cycleinternal combustion engine, having a two-stroke engine cycle, with aselectable combustible charge which charge is proportional to the torquedemand on the engine by varying the time in the cycle at which anexhaust valve closes, the exhaust valve being closed earlier in thecycle for high torque demand and closed later in the cycle for reducedtorque demand.
 4. The method of claim 3 including the additional step ofselecting the quantity of fuel to be ingested into the engine inaccordance with the time at which the exhaust valve is closed so as tomaintain a desired fuel to air ratio.
 5. In a two-stroke-cycle sparkignited engine, having a two-stroke engine cycle, and having a pistonreciprocable in a cylinder, said cylinder having an inlet port and saidcycle having intake, exhaust and compression segments; and a sparkigniter for igniting a combustible mixture in the cylinder during thecompression segment of the cycle, the improvement comprising:first meansfor compelling air under charging pressure through said inlet port intosaid cylinder during said intake segment; and second means comprising atleast one exhaust valve for said cylinder that is opened and closed by avalve actuator independently of engine crankshaft position forexhausting air from said cylinder during said exhaust segment of thecycle which exhaust segment overlaps at least a portion of said intakesegment whereby motion of said piston in said cylinder is substantiallyfree of any throttling vacuum thereby increasing engine efficiency. 6.The improvement of claim 5 wherein said second means includes controlmeans for providing control signals to said valve actuator to cause saidactuator to open and close said valve to obtain said increasedefficiency.
 7. In a two-stroke-cycle engine, having a two-stroke enginecycle, and having a piston reciprocable in a cylinder wherein the cyclehas intake, exhaust and compression segments, the improvementcomprising:first means for adjusting said exhaust segment to provide acompression ratio in said cylinder which has a predeterminedrelationship to engine torque and speed, the compression ratio for lowtorque being significantly lower than that for high torque and thecompression ratio for low speed being less than for high speed; andsecond means for admitting fuel to said cylinder during said compressionsegment.
 8. The improvement of claim 7 wherein said first means includesat least one exhaust valve for said cylinder that is opened and closedby a valve actuator to obtain said compression ratios independently ofengine crankshaft position.
 9. The improvement of claim 8 wherein saidfirst means includes means for sensing engine torque and speed demandsand for providing control signals to said valve actuator to cause saidactuator to open and close said valve to obtain said compression ratios.10. In a two-stroke-cycle spark ignited engine, having a two-strokeengine cycle, and having a multiplicity of pistons reciprocable inrespective cylinders, each cylinder having an inlet port and said cyclehaving intake, exhaust and compression segments; and spark igniters forigniting a combustible mixture in the respective cylinders during therespective compression segment of the cycle, the improvementcomprising:first means for compelling air under charging pressurethrough said inlet ports into each of said cylinders during respectiveintake segments; second means for exhausting air from said cylinderduring respective exhaust segments of the cycle which exhaust segmentsoverlay at least a portion of the corresponding intake segment wherebymotion of said piston in said cylinder is substantially free of anythrottling vacuum thereby increasing engine efficiency, said secondmeans comprising at least one exhaust valve for each of said cylindersthat is opened and closed by a respective valve actuator independentlyof engine crankshaft position; and control means for providing controlsignals to said valve actuator to cause said actuator to open and closethe corresponding at least one valve to obtain said increasedefficiency.
 11. In a two-stroke-cycle engine, having a two-stroke enginecycle, and having a multiplicity of pistons reciprocable in respectivecylinders wherein the cycle for each cylinder has intake, exhaust andcompression segments, the improvement comprising:first means foradjusting each of said exhaust segments to provide a compression ratioin each of said cylinders which has a predetermined relationship toengine torque and speed, the compression ratio for low torque beingsignificantly lower than that for high torque and the compression ratiofor low speed being less than for high speed; and second means ofadmitting fuel to each of said cylinders during said compressionsegment.
 12. The improvement of claim 11 wherein said first meansincludes at least one exhaust valve for each said cylinder that isopened and closed by a respective valve actuator to obtain saidcompression ratios independently of engine crankshaft position, andmeans for sensing engine torque and speed demands and for providingcontrol signals to each said valve actuator to cause said actuator toopen and close its respective valve to obtain said compression ratios.13. A two-stroke-cycle engine, having a two-stroke engine cycle, andhaving a piston with a plurality of piston rings reciprocable in aclosed ended cylinder; said cycle having intake, exhaust, compressionand ignition segments, the engine comprising:a fuel injector locatednear the closed end of the cylinder; a spark igniter located near theclosed end of the cylinder; at least one intake port located in thecylinder sidewall; means for supplying a charge of pressurized airthrough the intake port into the cylinder during the intake segment ofthe cycle; at least one exhaust valve located near the closed end of thecylinder; control means responsive to engine load and engine speed tovary the exhaust segment accordingly for opening the exhaust valve priorto the intake segment of the cycle and closing the exhaust valve at thecompletion of the exhaust segment whereby a sufficient charge of air hasbeen admitted into said cylinder to have substantially purged saidcylinder of the burned gases from the previous ignition segment of thecycle and to introduce substantially clean air into said cylinder; meansto admit fuel into the cylinder through the fuel injector after theexhaust segment of the cycle; means for energizing said spark igniterafter said fuel injector has admitted fuel into said cylinder to providein concert with the exhaust segment stratified fuel/air layers in saidcylinder with the richest fuel/air layer near the closed end of thecylinder and progressively less rich fuel/air layers away from theclosed end toward the piston with a layer of predominantly fresh aircovering the piston and piston rings whereby the piston and piston ringsare cleansed of previously burned gases and the combustion gases arequickly cooled by the progressively less rich and fresh air layers andthe fresh air layer mixed quickly wit the already ignited richerfuel/air layers due to the gaseous turbulence caused by the extremetemperature gradient after ignition to minimize hydrocarbon and NOXpollutants in the cylinder exhaust gases.
 14. The two-stroke-cycleengine of claim 13 wherein said control means comprises a valve actuatorfor opening and closing of said exhaust valve upon opening and closingsignals as determined from engine parameters including engine load andengine RPM.
 15. The method of operating a two-stroke-cycle internalcombustion engine, having a two-stroke engine cycle, comprising thesteps of initiating exhaustion of gaseous fluid from the enginecombustion chamber; admitting air to the combustion chamber; terminatingexhaustion from the combustion chamber to provide a predeterminedcompression ratio that varies with engine load and engine RPM wherebythe compression ratio for low torque is significantly lower than thatfor high torque and the compression ratio for low RPM is less than thatfor high RPM; and admitting fuel to the combustion chamber subsequent toterminating exhaustion.